Ensure config callback is always removed in dream overlay

Currently, when repeatOnLifecycle is cancelled, it will throw a
CancellationException which will cause the cleanup of the callback to be
skipped. This change wraps the cleanup in a finally block so that it
always runs.

Fixes: 297320149
Test: verified callback is now removed when dream exits using log statements
Flag: NA
Change-Id: Ic6c32df3969c7fddbc544e369551a7a52f4cdae1